Polymers frequently become flame-resistive on addition of phosphorus or halogen compounds or mixtures thereof. Some polymers are processed at high temperatures, for example at 250.degree. C. or higher. For this reason, many known flame retardants are unsuitable for such applications, since they are too volatile or insufficiently heat-resistant.
It is an object of the present invention to provide flame retardants which do not have the disadvantages of existing flame retardants. For example, the flame retardants shall be essentially non-volatile at elevated temperatures (e.g. at 250.degree. C. or higher), simple and inexpensive to prepare, exhibit good flame retardancy and have high temperature resistance at the elevated temperatures discussed above.